I've been thinking somewhat about the possibilites and came up with the following ideas:
A trilogy, with each movie depicting one of the Great Wars.
The first movie would show the formation of the Horde, it's corruption by the Burning Legion and the invasion of Stormwind. This would mean that the would end on a more or less sour note, however I can see the final scene being Anduin Lothir and the refugees cacthing a glimpse of Lordaeron city, as they sail towards it, full of hope, but also aware of the hard times that are still to come. Or the formation of the Great Alliance, ready to march south and recapture Stormwind. It would also be a nice cliffhanger.
The second movie would focus more on the adventures of the Sons of Lothar in Draenor, I think audiences would more easely relate to it. The movie would still show the last battle of the Second War, Lothar's death and Turalyon taking charge.
The last movie poses a problem because between Reign of Chaos and The Frozen Throne there's simply too much material for just one movie, I think. It could either focus on Arthas, Thrall or Illidan, though the fans would complain about not getting the full story.
There's also the alternative of starting right away with the story of the third game. It's the one people are more likely to remember and enjoy, due to its proximity and relevance to the setting of the MMO. There could still be some flashbacks showing stuff like the formation of the Horde, the past wars, but it would follow the narrative present in RoC and TFT. At least two movies would be required, I think.
The final alternative would be a movie based on Classic WoW, more specifically the Onyxia storyline. The movie would focus on Katrana Prestor's machinations, which would be eventually unveiled by a group of Alliance members.
In the meantime, Thrall sensing that something was not right with the Alliance leadership would send some trusted Horde members to investigate the issue as well. Both forces would eventually join up and try to defeat Onyxia in her lair.
The movie would more or less follow the structure of the attunement quest line.